Optical waveguides are typically solid conduits formed of light-transmitting material with a higher refractive index than that of its surroundings. Light enters an input of an optical waveguide and is typically reflected at the surfaces of the optical waveguide i.e., the interface between the solid material and the surrounding air. The internal reflection generally occurs along the length of the optical waveguide and tends to maintain light within the optical waveguide.
Waveguides in optical silica chips route light to different locations on the chip. Optical silica chips use planar technology restricting the paths of waveguides to two dimensions. In certain configurations, the paths of waveguides inevitably cross the paths of other waveguides on the same chip. Such crossings tend to result in optical losses caused by interfering cross talk between different waveguides at the junction of the crossing; commonly referred to as scattering. Losses tend to increase in direct proportion to the quantity of waveguide crossings.
Most practical solutions for dealing with losses associated with crossing of waveguides involve equalizing optical losses for the paths of the waveguides. Otherwise, imbalances may result with stronger optical signals overpowering weaker optical signals. One conventional solution used to equalize losses for waveguides is to insert “dummy waveguide crossings” across the paths of waveguides with fewer crossings so that each waveguide has an equal number of crossings. A dummy waveguide crossing is typically a small piece of a waveguide with the same geometric and material quantities of an actual waveguide, but is usually substantially shorter in length. The dummy waveguide is inserted across the path of an actual waveguide for the purpose of masquerading as an actual crossing to introduce the same loss characteristics as an actual waveguide crossing.
Equalizing the quantity of crossings for each waveguide tends to equalize the losses for all the waveguides. Introducing dummy waveguide crossings, however, tends to penalize waveguides with fewer crossings than the waveguide(s) with the most crossings. In other words, by making all the waveguides collectively resemble the worst-case waveguide's path in terms of the number of crossings and total optical loss, one penalizes waveguides whose paths have less actual crossings and hence less optical losses. For example, suppose that the worst-case waveguide's path has five crossing and another waveguide path has only one crossing. To equalize the quantity of losses between the two paths, four dummy waveguide crossings need to be added to the path with only one crossing so that it resembles the waveguide with five crossings.
To complicate matters further, dummy waveguide crossings are not precise and the losses created by them can vary depending on the spacing used between them. That is, the total loss imparted on a waveguide by dummy waveguide crossings may not equal the same loss imparted to the waveguide by actual crossings, depending how far apart each dummy waveguide crossing is spaced. For example, the total loss after a waveguide's path passes through N crossings may actually equal N−1 crossing if the spacing between dummy waveguide crossings is not chosen correctly.